Electrochemical Sensing For Heavy Metal Ions Based On The Signal Amplification Of ExoⅢ And Functionalized MoS₂,H_xTiS₂ Nanosheets

Heavy metal pollution,as a serious and widespread environmental problem,is making acute threats to human health.Heavy metal ions in the environment could enter tissues and organs of the human body via the food chain’s bioaccumulation,which result in the severe and permanent damage even if the human body has suffered from chronic exposure;therefore,to develop the highly effective and sensitive detection methods is the basis for protecting the living environments and preventing the damage of heavy metal pollution.Compared with traditional analytical techniques,electrochemical sensing methods show many advantages such as simple instrumentation,low time-consumption and real-time detection in situ.In this thesis,in order to obtain the higher sensitivity,four types of electrochemical sensing methods were constructed based on the signal amplification of exonuclease III(ExoIII),functionalized-hydric titanium disulfide(HxTiS2)and-MoS2 nanosheets,which realized the highly sensitive and selective detection of Hgig,Cu2+,and Cd2+,respectively.In addition,the mechanism of selective recognization between electrochemical sening elements and heavy metal ions were explored.The main research contents include four aspects as follows:(1)The combination of thymine-Hg2+-thymine(T-Hg2+-T)coordination chemistry and the high effectivity and specificity of exonuclease III(Exo III)in digesting DNA leads to the cycle process of DNA hybridization and enzyme digestion,which realize the recycling signal amplification and have the unique advantages for detecting Hg2+ with an ultralow concentration.Single-strand probe DNA was immobilized on an Au electrode via Au-S covalent bonds.In the presence of Hg2+,the probe DNA hybridized with the target DNA containing four thymine-thymine(T-T)mismatches via the Hg2+-mediated coordination of T-Hg2+-T base pairs,and generated DNA duplex with a special structure.Then the probe DNA in the as-formed DNA duplex was specifically recognized and selectively digested by Exo III,while the target DNA was safely dissociated from the DNA duplex to subsequently hybridize with a new signal probe,leading to target recycling and signal amplification.As a result,the peak current caused by the electrostatic interactions of[Ru(NH3)6]3+ cations as the redox indicator with the backbone of the probe DNA decreased by different degrees,corresponding to the Hg2+ concentrations,which could be used to detect Hg2+.Under the optimum conditions,the proposed electrochemical sensing method showed a robust detection limit as low as 1 pM(S/N=3),with a wide linear range from 0.01 to 500 nM and a good selectivity,which offer a novel route for constructing electrochemical biosensor for ultralow Hg2+ detection.(2)Hydrogen-incorporated TiS2(HxTiS2)ultrathin nanosheets with high electrical conductivity were prepared by the lithium intercalation and exfoliation method.Exploring the influences of the incorporation of HxTiS2 ultrathin nanosheets on the morphology and electrochemical properties of HxTiS2 nanosheet-polyaniline(PANI)nanocomposites synthesized by the absorption-polymerization reactions of aniline on the surface of HxTiS2 nanosheets.After the experiment optimization,the results demonstrate that as the mass ratio of HxTiS2 nanosheet versus aniline monomer is 1:1.5,the resultants are three-dimensional porous HxTiS2 nanosheet-PANI nanocomposites that as the electrode material displayed the larger heterogeneous electron-transfer rate,consequently the signal amplification from both component and structure could be realized.Via the coordination interaction between Cu2+cations and the imine moieties,a novel electrochemical sensing method based on three-dimensional porous HxTiS2 nanosheet-PANI nanocomposites was successfully developed to directly detect trace Cu2+.Furthermore,FT-IR spectra and UV-vis absorption spectra were employed to explore the speculative mechanism of selectively detecting Cu2+.Under the optimal conditions,the as-prepared electrochemical sensing method exhibited highly sensitive and selective sensing performances with a detection limit of 0.7 nM(S/N=3)and a linear range from 25 nM to 5 μM.Furthermore,the proposed mechanism of selectively detecting Cu2+ and the synthesis method of three-dimensional porous nanosheet-conductive polymer nanocomposites would lay the foundation for the design of sensing element for Cu2+detection and the applications of two-dimensional nanomaterials in electrochemical sensing fields.(3)Via Au-S covalent bonds,p-aminothiophenol(PATP)was anchored on the surface of nanocomposites of Au nanoparticle and hydrogen-incorporated TiS2 nanosheets(PATP/Au-HxTiS2).Based on the coordination interaction between Cu2+ cations and the imine moieties generated after PATP electrooxidation,a novel electrochemical sensing method based on PATP/Au-HxTiS2 nanocomposites for directly detecting trace Cu2+ was successfully developed.After the experiment optimization,the results demonstrate that as the volum ratio of HAuCl4 versus HxTiS2 nanoshees was 4:1.5,the plane edges of HxTiS2 nanosheets could offered Au nanoparticles with incubation sites for the heterogeneous nucleation and growth.In return,Au nanoparticles were not only served as an excellent electrical catalyst to facilitate electron transfer and as a substrate for loading more PATP molecules,but also could effectively avoid the aggregation of HxTiS2 nanosheets,which realized the synergetic effects from Au-HxTiS2 for the signal amplification.FT-IR spectra and UV-vis absorption spectra were employed to chatacterize the PATP/Au-HxTiS2 nanocomposites before and after detecting Cu2+ to illustrate the mechanism of selectively detecting Cu2+.Under optimal conditions,the as-formed electrochemical sensing system exhibited highly sensitive and selective sensing performances with a detection limit of 90 pM(S/N=3)and a linear range from 0.2 nM to 5μM,which would broaden the applications of HxTiS2 nanosheets in environmental monitoring.(4)By liquid phase exfoliation(LPE)method,MoS2 nanosheets functionalized by solvent molecules were synthesized in N,N-dimethyl formamide(DMF),ethylene glycol,ethanol,formamide,and N-methyl-2-pyrrolidone(NMP),respectively.In addition,the influences of the solvent parameters on the morphologies,the electrocatalyitic activity and the cytotoxicity were systematically investigated.After screening the MoS2 nanosheets with higher electrocatalyitic activity and lower cytotoxicity,a novel electrochemical sensing method based on MoS2 nanosheets functionalized by solvent molecules for directly detecting trace Cd2+ was successfully developed according to Pearson hard-soft,acid-base(HSAB)theory.In addition,the signal amplification for Cd2+ detection was realized by virtue of the high electro-activity and specific surface areas of MoS2 nanosheets functionalized by solvent molecules.Density functional theory(DFT)calculations were employed to investigate the speculative mechanism of selectively detecting Cd2+.Under the optimum conditions,the proposed electrochemical sensing method for Cd2+ detection showed a detection limit of 0.2 nM and a linear range of 2 nM-20μM,which would offer a novel route for constructing electrochemical sensors based on MoS2 nanosheets.